Pressure wave transmission arrangements



March 1, 1966 H. J. ROUND ET AL 3,238,498

PRES SURE WAVE TRANSMI S S ION ARRANGEMENTS Filed April 20, 1962 2 Sheets-Sheet l l l I I Z\ PRESSURE WAVE '2 TRANSDUCER I I l sH/Ps HULL INVENTORS m Jimmy Ha/nZw .BY a:

ATToraN EYS March 1, 1966 H. J. ROUND ETAL 3,238,498

PRESSURE WAVE TRANSMISSION ARRANGEMENTS Filed April 20, 1962 2 Sheets-Sheet 2 /o// l a a 32 4 3mm Q 99 44 5 44 /O IO a a 9 NH 9 9 55 36 0 /o FIG-8. 44

lNVENTORS z/M W M Ala/MM .3) 611W x United States Patent C This invention relates to pressure wave transmission arrangements and more particularly to arrangements, such as are used in echo-Sounders, for transmitting pressure waves in water.

Customary present day practice in echo sounders is to form in the ships hull a hole providing communication between the sea and the interior of a sea-filled tank built on to the inside of the hull and to mount the pressure- Wave transmitting transducerusually a magneto-stric- H 'tive deviceinside the tank so that pressure waves are transmitted by the transducer direct to the Water in the tank and pass through the hole in the hull to the sea outside. Such an arrangement, though efiicient from the point of view of pressure-wave transmission, is objectionable from the practical point of view in that it requires the provision of a hole in the hull, and since the transducer is in" a tank in direct communication with the sea,

the transducer is not accessible for servicing or replacement without getting the ship out of the water unless (as is commonly done) means are provided for closing otf the hole. The provision of such closure means add to the expense as, of course, does docking the ship.

It is well known that if the hole in the ships hull be dispensed with and a water tank containing the transducer is built on to the inside of the hull so that the said hull forms the bottom of the tank, pressure waves transmitted by the transducer to the water in the tank will be transmitted by the hull itself to the outside sea. This, however attractive it is structurally, is seldom done in prac- 'tice because of the very heavy-indeed prohibitively heavy-losses in pressure wave transmission which occur with known arrangements of this nature. To quote practical figures, where the operating oscillatory frequency is of the order of 50 kc./s. the interposition of a steel hull plate about /2" thick in the pressure wave transmission path from the transducer to the outside sea will cut down the transmission to about /5 of that which occurs with direct unobstructed transmission. In this connection it i should be noted that echo sounding involves both transmission and reception so' that reduction of loss in transmission is accompanied by reduction of loss in reception. The present invention seeks to avoid this defect and to provide improved pressure wave transmission arrangements whereby pressure waves, produced on the inner side of a ships hull, will be transmitted to the outside sea through the material of the hull without serious or unacceptable loss of transmission efiiciency.

According to this invention a ship-borne pressure wave transmission arrangement comprises a pressure-wave transducer inside the metal hull of the ship; at least two plates spaced each a short distance froman unbroken portion of the hull, one on one side thereof and the other on the other, said plates being of less thickness than that of said hull portion, and the spaces between the hull and said plates, and between the plates (if there be more than one) on the same side of the hull portion, having fillings of material having at least approximately the same density and elasticity as water; and means for transmitting pressure waves from the transducer to the outside sea (or vice versa) through the unbroken portion Patented Mar. 1, 1966 of the hull and the plates and fillings on the two sides thereof, the whole arrangement being such that said unbroken hull portion and plates and fillings act as a coupled band-pass system for the pressure waves. In the analogue coupled band pass system the masses of the hull portion and of the plates act as the series inductanees of the band pass system and the compliances of the space fillings act as the shunt capacitances. The analogues of mechanical resistance, velocity and pressure are, respectively, electrical resistance, electrical current and voltage.

The pressure wave transducer may be in a tank which is adapted to be filled with water and is on the inside of the hull, the unbroken portion of the hull constituting the bottom of the tank with the inner spaced plate or plates and filling or fillings above it.

Alternatively the transducer may be arranged to transmit vibrations directly to (and receive vibrations directly from) the plate inside the hull or, if there be more than one plate inside the hull, the innermost of those plates.

Advantage may be had by fitting the spacing fillings under initial pressure exceeding that which would be caused in the filling material by pressure waves alone.

Although considerable improvement in pressure wave transmission efiiciency as compared with a known arrangement in which there is merely the hull thickness between a transducerand the outside sea can be obtained by providing only two spaced plates, one each side of the hull, it is still better to provide at least four plates, two (or more) on each side of the hull, each separated from the adjacent place or hull (as the case may be) by a short distance, the plate thickness increasing from the outermost plates towards the hull which is thicker than any of the plates.

The space fillings may be actually of water but are preferably wholly or mainly of rubber or like material of approximately the same density and elasticity as water. This is preferred for reasons of structural convenience, it being very convenient to constitute the space fillings by layers of said rubber or like material cemented to the metal of the plates and/or hull, it is also very easy and convenient to put rubber or rubber-like space fillings under initial pressure by compressing them between the metal members between which they are situated.

For optimum results the plate thicknesses and separating space thicknesses should be chosen with care. Although it is possible, in any given case of known operating frequency and known hull thickness, to calculate optimum thicknesses, such calculation is not easy and it is at present considered better to choose the thicknesses by experiment. As a guide the following experimental results obtained with an operating frequency of 50 kc./s. and a hull plate /2 thick are given:

1) With only the hull between the water in the transducer-containing tank and the outside sea, the pressure wave transmission was cut down to about 20% of that obtained with unobstructed transmission, i.e., with no interposed hull in the transmission path.

(2) By providing on each side of the hull a steel plate 43 thick, parallel to the hull and spaced therefrom by a water-filled separating space approximately thick, the transmission was increased to about of the unobstructed transmission.

(3) By providing outwardly of the two plates mentioned in 2 above, two further plates, each separated from one of the two plates mentioned in 2 above by a water filled space A" thick and each A thick, the transmission was further increased to about of the unobstructed transmission.

The dimensions of the plates in the transverse direction, i.e., in directions at right angles to that of the intended pressure wave propagation, are quite uncritical,

it being enough to ensure that they are big enough to lie in the way of the propagated waves.

The invention is illustrated in and further explained in connection with the accompanying drawings. For convenience of reference the figures are numbered consecutively. In the drawings, FIGURE 1 is a schematic sectional view of one embodiment; FIGURE 2 is a highly simplified representation, provided for purposes of explanation, of one embodiment; FIGURES 3, 4 and 5 are analogue electrical coupled band pass systems; FIGURES 6 and 7 are mutually perpendicular views, the former in section illustrating one method of applying pressure to a solid space fitting; FIGURE 8 is a view of the same general nature as that of FIGURE 1 showing a modification; and FIGURE 9 is the analogue electrical circuit of FIG- URE 8.

Referring to FIG. 1, 1 is a portion of the steel hull of a ship on the inside of which is mounted a water filled closed tank, the sides and top of which are referenced 2 and the bottom of which is constituted by the portion 1. In the tank is mounted a magneto-strictive or other pressure wave transducer represented schematically at 3 and arranged to direct its waves towards the portion 1. As so far described the apparatus is as well known, except for the omission of any hole in the hull, and needs no further description. Two steel plates 4 and 44 are mounted parallel to the hull on each side thereof, i.e., one inside and the other outside, being spaced therefrom by interposed layers 5 and 55 cemented to the hull and to the appropriate plate. These layers are made of rubber or other suitable materialfor example the material known as Rho-C rubberchosen to have approximately the same elasticity and density as water. Outwardly of the plates 4 and 44 are two further steel plates 6 and 66 parallel to the plates 4 and 44 and similarly spaced and held in position by layers 7 and 77 of rubber or other material of the aforesaid characteristics cemented to the plates 4 or 44 and 6 or 66.

The hull thickness is greater than that of the plates 4 and 44 which in turn is greater than that of the plates 6 and 66 and the thickness of the layers 7 and 77 is greater than that of the layers 5 and 55. The relative thicknesses of hull, plates and layers shown in the drawing have been found to give good results in a case Where the operating frequency was 50 kc./s. and the hull was /2" thick. Except as regards these relative thicknesses the drawing is, however, purely schematic.

It is believed that the plates and interposed layers on the two sides of the hull act in conjunction therewith as a coupled band-pass system giving approximate impedance matching with the water at both ends (i.e., in the tank and the sea) and built up of masses (the plates and the hull) and compliances (the layers).

This will be better understood from a simple case chosen for detailed explanation with reference to FIGS. 2 and 3. In FIG. 2 the hull plate 1 is /2" thick and there are two steel plates 4, 44 each /s" thick, one on each side thereof and spaced therefrom by compliance spaces 5, 55 each thick. X-X is the central plane and with plates 4 and 44 are presumed to be in water, the former inside the ship in a tank in which the transducer is situated and the latter outside the ship. The measured transparency at 48 kc./s. was found to be approximately 70% of the unobstructed transmission. FIG. 3 shows the nature of an acceptable equivalent electrical circuit. The central large inductance 4L represents the central large mass provided by the hull portion 1, the smaller inductances L represent the masses provided by the plates 4 and 44 and the two condensers C represent the compliances at 5 and 55. The resistances R represent mechanical resistance. Water resistance loading is equal to 150,000 mechanical ohms/ sq. cm. Consider an area of one square centimetre transverse to the direction of propagation. The weight of this area of Vs" plate is 2.5 grammes and that of 1 sq. cm. of A4" plate is grammes. At a frequency of 48 kc./s., W=27rf 300,000 so that in mechanical units the dimensions become as in FIG. 4. In FIG 4 R has been taken as 150,000 ohms, i.e., equal to the water loading resistance. This choice of R is arbitrary but convenient for easy measurement of quantities. The corresponding electrically dimensional equivalent circuit, divided by 1,000 for the sake of convenience, is shown in FIG. 5. Voltage applied at point P of FIG. 5 is equivalent to pressure applied to the plate 4, and since this voltage will be (approximately) repeated at point S, pressure applied to plate 4 will be (approximately) equivalent to pressure applied to the sea. The voltages at points P, Q, R and S of FIG. 5 are relatively representable by the values 1, 5, 5, 1 respectively so that any pressure on plate 4 results in 5 times that pressure in each of the compliances 5 and 55. In other words the Q of the circuit may be considered as being 5.

Measurement of transparency with increasing values of applied pressure in plate 4 showed that non-linearity began to appear with output pressures well below the pressure of cavitation. As will be seen from considerationsof FIG. 5, cavitation will first appear in the compliance gaps 5 and 55 and the limiting pressure which can be applied at plate 4 is Pc/S, where P0 is the cavitation pressure in the said gaps. In other words the pressure limit is set not by cavitation in the sea but by cavitation in the compliance gaps. It appears, however, to be clear that the arrangement acts as a coupled band-pass system between the plate 4 and the compliance 5 on the one hand and plate 44 and compliance 55 on the other, the hull acting as the mutual inductance. From this follows the important result that hull vibrationa potent cause of false echoes in known echosounders in which the hull is vibratedis much reduced because whereas in a known system in which hull vibrations are transferred directly to the sea the velocity in the hull is the same as that in the sea, in this system the hull velocity is reduced to l/ Q of the velocity in the sea and in plate 44 where Q is the Q value of the end section of FIG. 5, i.e., the section including the inductance R-S and one resistance R.

The pressure applied to the plate 4 can be greatly increasedbey0nd that at which cavitation occurs in the seaby using suitable solid material, such as so-called Rbo-C rubber as already described stuck on with an epoxy resin, instead of water for the compliances 5 and 55. Still further substantial improvement can be obtained by arranging the compliance material to be under initial positive pressure exceeding that produced dynamically by pressure waves. Good results are obtainable by using ordinary waterproof grease under pressure as the compliance material. FIGS. 6 and 7 are partial views showing one way of applying pressure to a compliance filling made of solid material. Threaded studs 8 are spotwelded to the hull plate 1 and pass through clearance holes giving good clearances in a rubber-like compliance filling sheet, here exemplified as the sheet 55, and the side plate exemplified as the plate 44. Nuts 9 screwed on to the projecting ends of the studs apply the required pressure through metal and rubber washers 10 and 11.

It is not necessary to apply the pressure waves from the transducer through the water in an internal water tank as shown in FIG. 1 and, with correct design, substantial advantages are obtainable by direct application of pressure waves from the transducer to the innermost plate and vice versa. A nickel cored magneto-strictive transducer will deliver greatest power with a high Q valve, the limitation from this point of view being that set by permissible flux density in the nickel. Too high a Q value should be avoided since it will present the transmission of side bands necessary for the obtaining of sharp echoes. In practice Q values of between 10 and 20 are usually adopted for nickel cored magneto-strictive transducers. A practical figure for the Q value of a transducer in contact with and transmitting through water is about 18. Clearly, With an arrangement as illustrated in FIG. 1 in which the transmission is through the water in the tank contituted by the hull 1 and the walls 2, cavita-. tion in the Water will set a limit. This limit can be avoided by transmitting the pressure waves directly to the innermost plate, i.e. the plate 6 of an arrangement as shown in FIG. 1.

An arrangement with such direct transmission from the transducer is illustrated in FIG. 8 which for simplicity is exemplified as one with only two side plates and compliance fillings. Here the transducer is shown schematically as of the quarter wave length rod type with a permanent magnet 31, two quarter wave length rods 32 and windings 33. The rod ends are spot welded to a suitably chosen part of the innermost plate 4 and transmit vibrations directly to and receive vibrations directly from said plate 4 without going through water. Of course, for best results, good impedance matching between the transducer and the plate 4 should be obtained by suitable design in accordance with known principles. In this way more power and pressure can be delivered to plate 4 than with a comparable water tank arrangement as illustrated in FIG. 1. With an arrangement as illustrated in FIG. 8 the Q value will be mainly determined by the energy flowing into the sea. FIG. 9 is an equivalent circuit for the arrangement of FIG. 8. Here the section T is the equivalent of the transducer, L being the equivalent inductance and C the equivalent capacitance. As will be seen, the section T replaces one of the resistances R of FIG. 3. If C is about A C and L approximately equals L the eifective Q value given to the transducer will be about 20 and a series resistance effect of about 150 ohms will be given to the transducer.

We claim:

1. A ship-borne pressure wave transmission system comprising a pressure-wave transducer inside the metal hull of the ship; and a coupled band-pass system, said band-pass system comprising at least two plates spaced each a short distance from an unbroken portion of the hull, one plate on one side of said portion and the other plate on the other side of said portion, there being spaced plate and hull surfaces on each side of the hull, said plates being of less thickness than that of said portion, and said spaces having fillings of material having at least approximately the same density and elasticity as water; and means for transmitting pressure waves from the transducer to the outside seat and from the outside seat to the transducer through the unbroken portion of the hull and the plates and fillings on the two sides thereof.

2. A system as claimed in claim 1 including a tank, and wherein said transducer is in said tank which is adapted to be filled With water and is on the inside of the hull, the unbroken portion of the hull constituting the bottom of the tank with the inner spaced plate or plates and filling or fillings located above said portion.

3. A system as claimed in claim 11 wherein the transducer is arranged to transmit vibrations directly to the innermost plate inside the hull.

4. A system as claimed in claim 1 wherein the fillings are under initial pressure exceeding that which would be caused in the filling material by pressure waves alone.

5. A system as claimed in claim 3 wherein the filling outside the hull is under initial pressure exceeding that which would be caused in the filling material by pressure waves alone.

'6. A system as claimed in claim 1 and including four plate, two on each side of the hull, each separated from the adjacent plate or hull (as the case may be) by a short distance, the plate thickness increasing from the outermost plates towards the hull which is thicker than any of the plates.

7. A system as claimed in claim 1 wherein the fillings are of water.

8. A system as claimed in claim 1 wherein the fillings are of waterproof grease.

9. A system as claimed in claim 1 wherein the fillings are of rubber or rubber-like material.

References Cited by the Examiner UNITED STATES PATENTS 4/1946 Turner 340-8 11/1960 McMillan 340-8 OTHER REFERENCES CHESTER L. JUSTUS, Primary Examiner.

J. W. MILLS, J. P. MORRIS, Assistant Examiners. 

1. A SHIP-BORNE PRESSURE WAVE TRANSMISSION SYSTEM COMPRISING A PRESSURE-WAVE TRANSDUCER INSIDE THE METAL HULL OF THE SHIP; AND A COUPLED BAND-PASS SYSTEM, SAID BAND-PASS SYSTEM COMPRISING AT LEAST TWO PLATES SPACED EACH A SHORT DISTANCE FROM AN UNBROKEN PORTION OF THE HULL, ONE PLATE ON ONE SIDE OF SAID PORTION AND THE OTHER PLATE ON THE OTHER SIDE OF SAID PORTION, THERE BEING SPACED PLATE AND HULL SURFACES ON EACH SIDE OF THE HULL, SAID PLATES BEING OF LESS THICKNESS THAN THAT OF SAID PORTION, AND SAID SPACES HAVING FILLINGS OF MATERIAL HAVING AT LEAST APPROXIMATELY THE SAME DENSITY AND ELASTICITY AS WATER; AND MEANS FOR TRANSMITTING PRESSURE WAVES FROM THE TRANSDUCER TO THE OUTSIDE SEAT AND FROM THE OUTSIDE SEAT TO THE TRANSDUCER THROUGH THE UNBROKEN PORTION OF THE HULL AND THE PLATES AND FILLINGS ON THE TWO SIDES THEREOF. 